Plating, implants and similar shouldn't set off store alarms, the detection gates are a type of proximity based RFID, which only work when an active tag gets closer to one gate than the established and existing distance between the two gates. They're not metal detectors otherwise they'd go off constantly when people come in and out with trolleys, car keys etc. The most likely explanation is that there has coincidentally been a tag nearby when you've gone through. (The tag does not necessarily need to be between the gates to cause an alarm). It could also be that there's still an active tag on you somewhere and you've not realised. The tags can of course be those big whacking plastic ones but they can also simply be a sticker with a thin circuit embedded. Check your clothing and coat labels for something hidden.